Initial acceleration?

I have been using mousetrap plugin for a while and it is sooo awesome. I am trying to study the influence of proactive inhibition on mouse movement. Could you please help me with devising a way to calculate initial acceleration? I.e. acceleration on the first recorded section of the trajectory?

I have another (tangentially related) question. In one of the previous theme (long ago) you provided the way to calculate the distance after certain amount of time (for example). Is there any way to make it trial by trial?

In our stop-signal task, the stop signal sounded after a delay that was set using a staircase procedure in each trial. Suppose we want to calculate distance that an individual went in each stop trial after hearing a stop-signal. Is there a way to calculate this given the stop-signal delay in a given trial(i.e., individually for each trial).

regarding your first question ("What I have in mind is first 25% of trajectory. Is it possible to compute maximum AND averafe acceleration on this part?"):

I think there are two ways how you could to do this (not sure which way is best). I'll illustrate both ways with R scripts that use the example data in mousetrap. Please note that this is just an illustration how you can implement it on a technical level - I have not thought thoroughly about whether this procedure makes sense in practice.

First variant:

Time normalize trajectories first, the calculate the acceleration for each point and then aggregate (manually via the apply function) over the first 25 % of values per trial:

Regarding the second question ("In our stop-signal task, the stop signal sounded after a delay that was set using a staircase procedure in each trial. Suppose we want to calculate distance that an individual went in each stop trial after hearing a stop-signal. Is there a way to calculate this given the stop-signal delay in a given trial(i.e., individually for each trial)."):